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Fishing fleets dump about 10 per cent of the fish they catch back into the ocean in an "enormous waste" of low-value fish despite some progress in limiting discards in recent years, scientists said on Monday.

A decade-long study, the first global review since 2005 and based on work by 300 experts, said the rate of discards was still high despite a decline from a peak in the late 1980s. Discarded fish are usually dead or dying.

Almost 10 million tonnes of about 100 million tonnes of fish caught annually in the past decade were thrown back into the sea, according to the "Sea Around Us" review by the University of British Columbia and the University of Western Australia.

Industrial fleets often throw back fish that are damaged, diseased, too small or of an unwanted species. A trawler with a quota only to catch North Atlantic cod, for instance, may throw back hake caught in the same net.

Discards are an "enormous waste…especially at a time when wild capture fisheries are under global strain amidst growing demands for food security and human nutritional health," they wrote in the journal Fish & Fisheries.

Declining numbers

The report welcomed the decline in discards from a peak of about 19 million tonnes in 1989, roughly 15 per cent of a total catch of 130 million tonnes.

The fall may be linked to restrictions in some nations on discards and improved fishing gear. Also, a rise in the price of fishmeal for aquaculture made it profitable to keep formerly low-value species, it said.

But it might just reflect a lack of fish.

"We suspect that (the decline) is because overfishing…has already depleted the species being discarded," lead author Dirk Zeller of the University of Western Australia told Reuters.

Researchers from the University of British Columbia and the University of Western Australia suspect that overfishing may be responsible for the decline of fish thrown back into our oceans. (Emilio Morenatti/Associated Press)

Few fish survive getting thrown back although some species such as sharks, rays or crustaceans are more resilient.

The UN's Food and Agriculture Organization last estimated, in 2005, that eight per cent of fish were discarded from 1992–2001. Those numbers, using different methods, are not directly comparable with the Sea Around Us data.

The scientists said discards were now highest in the Pacific, a shift from the Atlantic.

Russian fleets, for instance, discarded large amounts of Alaska pollock in the North West Pacific because they only wanted the roe. Fleets from South Korea, Taiwan and China were also among those dumping Pacific fish.